The present invention relates to a technique of mounting a chip component on a wiring board and, more particularly, to a chip component mounting board on which a chip component is to be mounted, a chip component mounting structure, and a method of manufacturing the chip component mounting board.
In recent years, various techniques of directly mounting a semiconductor chip on a board have been developed to realize ultra-high-density mounting of devices. Of these techniques, bare chip mounting is known as a technique having a lot of advantages such as ultra-low-profile mounting depending on only the board thickness and the semiconductor chip thickness, a small mounting area depending on only the semiconductor chip size, and excellent electrical characteristics due to the shortest connection. Examples of the bare chip mounting technique are flip chip mounting in which solder bumps are formed on the electrodes of a bare chip, and the bare chip with its wiring surface facing downward is mounted on the wiring board, and beam lead chip mounting using beam leads of gold or platinum in place of bumps.
In the above-described flip chip mounting, generally, after the bare chip is mounted on the wiring board, a resin is injected into a gap between the bare chip and the wiring board and cured. When the resin is cured, the bare chip and the wiring board are integrated. The stress applied to the connection portion between the bare chip and the wiring board is dispersed, and airtightness to the outer atmosphere at the connection portion increases, resulting in high reliability.
As a method of injecting the resin into the gap between the bare chip and the wiring board, the resin is injected from the gaps at the four corners or four sides of the rectangular bare chip mounted on the wiring board such that the applied resin flows toward the center of the gap between the bare chip and the wiring board due to capillarity.
When the resin is to be injected from the gaps at the four corners or four sides of the rectangular bare chip, the resin spreads in a short time. However, the interface between the resin and the bare chip and that between the resin and the wiring board partially have portions where the resin does not spread. These portions where the resin does not spread are called voids.
If the resin is injected from not the four corners or four sides but two corners or two sides of the bare chip to prevent voids, a long time is required to spread the resin, so the injection time is prolonged. Conventionally, when the resin is to be injected in a short time while preventing voids, a bare chip mounting board as shown in FIGS. 18A and 18B is used.
FIG. 18A shows a conventional bare chip mounting board, and FIG. 18B shows a section taken along a line II--II in FIG. 18A. A bare chip mounting portion 31 indicated by an alternate long and two short dashed line in FIG. 18A is a virtual area where a bare chip is mounted. As shown in FIG. 18B, in the conventional bare chip mounting board, a plurality of board pads 24 to be connected to the bare chip are formed on a wiring board 30 in correspondence with the electrode positions of the bare chip, and a solder resist 22 is formed on the wiring board 30 having the board pads 24.
As shown in FIG. 18A, to connect each board pad 24 to a corresponding electrode of the bare chip, the solder resist 22 has four rectangular opening portions 21 which are parallel to the four sides of the bare chip mounting portion 31 in correspondence with the electrode positions of the bare chip. A through hole 23 through which the resin and air flow in resin injection is formed at the center of the bare chip mounting portion 31, as will be described later.
FIG. 19 shows a state wherein the bare chip is mounted on the bare chip mounting board shown in FIGS. 18A and 18B. As shown in FIG. 19, to mount a bare chip 26 on a bare chip mounting board 32, bumps 28 are formed on aluminum electrodes 27 of the bare chip 26, and the bare chip 26 with its upper surface facing downward is placed on the bare chip mounting board 32. After the bumps 28 and the board pads 24 are positioned, the bumps 28 are heated and fused.
When the temperature is reduced to harden the bumps 28, the bare chip 26 is bonded to the bare chip mounting board 32. After the bare chip 26 is mounted on the bare chip mounting board 32, a resin is pressurized and injected from the through hole 23 formed in the bare chip mounting board 32. FIG. 20 shows a state wherein the gap between the bare chip mounting board 32 and the bare chip 26 is filled with the resin.
To fill the gap between the bare chip mounting board 32 and the bare chip 26 with a resin 29, as shown in FIG. 20, another method may be used. In FIG. 20, the resin 29 is potted on the surface of the bare chip mounting board 32 on which the bare chip 26 is mounted. The interior of the structure is evacuated from the other surface of the bare chip mounting board 32, which is on the opposite side of the potting side, through the through hole 23 to draw by suction the resin 29 supplied to the bare chip 26 side and inject the resin 29 into the gap between the bare chip mounting board 32 and the bare chip 26.
With any method, the resin 29 can be pressurized and injected or drawn by suction using the through hole 23 and injected in a short time without generating any voids. The injected resin 29 is cured, so the bare chip mounting board 32 and the bare chip 26 are integrated.
However, according to the above-described method, since the through hole 23 is formed in the bare chip mounting board 32, the wiring arrangement on the bare chip mounting board 32 is restricted. In addition, the wiring space decreases, and the wiring efficiency lowers.
FIG. 21 shows another example of the conventional bare chip mounting board for flip chip mounting. Referring to FIG. 21, a bare chip mounting board (to be simply referred to as a board hereinafter) 41 has, on its insulating layer 41a, connection pads 44 connected to a wireless semiconductor chip (to be simply referred to as a semiconductor chip hereinafter) 42, i.e., a bare chip. The semiconductor chip 42 having a circuit device surface 42a facing downward is mounted on the board 41. At this time, the semiconductor chip 42 is connected to the connection pads 44 on the board 41 via solder bumps 43 formed on the circuit device surface 42a and having a height of 30 to 50 .mu.m.
The gap between the board 41 and the semiconductor chip 42 is filled with a sealing resin 45 consisting of an epoxy resin. The sealing resin 45 is also applied around the semiconductor chip 42 to seal the circuit device surface 42a of the semiconductor chip 42.
Since the semiconductor chip 42 is encapsulated with the sealing resin 45, water can be prevented from reaching the circuit device surface 42a. When a stress due to thermal expansion is applied to the semiconductor chip 42, the stress is dispersed over the lower surface of the semiconductor chip 42 to reduce the stress per solder bump 43. With this structure, the humidity resistance of the semiconductor chip 42 and the connection strength between the board 41 and the semiconductor chip 42 increase.
In the above-described conventional bare chip mounting board, however, the gap between the board 41 and the semiconductor chip 42 is as small as 30 to 50 .mu.m equal to the height of the solder bump 43. For this reason, if a circuit opposing portion 41b of the board 41, which opposes the circuit device surface 42a when the semiconductor chip 42 is connected to the connection pads 44, is warped to project upward, as shown in FIG. 22, the circuit device surface 42a of the semiconductor chip 42 may contact the surface of the board 41 in mounting the semiconductor chip 42 on the board 41 to destroy the circuit device of the semiconductor chip 42.
Even in use of a beam lead chip, the gap between the board 41 and the semiconductor chip 42 is small. In such a case, normally, destruction of the semiconductor chip 42 cannot be recognized until the operation confirmation process after the assembly of the board 41. Not only the semiconductor chip 42 but also the board 41 on which the semiconductor chip 42 is mounted is defective, resulting in a serious problem in production management.
In addition, injection of the sealing resin 45 takes a long time because of the small gap between the board 41 and the semiconductor chip 42. Furthermore, since the semiconductor chip 42 can hardly be uniformly encapsulated, bubbles may be formed in the sealing resin 45 to generate voids. If many voids are formed in the sealing resin 45, the humidity resistance and connection strength degrade. Use of an epoxy resin as the sealing resin 45 makes the above problem more serious because of the high viscosity of the epoxy resin.
Even when the gap between the board 41 and the semiconductor chip 42 is filled with the sealing resin 45, water from the semiconductor chip 42 side may permeate through the thin sealing resin 45 and reach the circuit device surface 42a of the semiconductor chip 42. Additionally, the connection strength between the semiconductor chip 42 and the board 41 is low.